1. Field of the Invention
The present invention relates generally to a stabilized current output circuit, and is directed more particularly to a constant current output circuit which can produce an output current stable even when temperature is varied upon being driven at a low voltage.
2. Description of the Prior Art
In the art, as to a transistor circuit which will produce a constant current, there have been proposed current mirror circuits such as shown in FIGS. 1 and 2.
In the prior art current mirror circuit shown in FIG. 1, there is provided an NPN-type transistor 1 which is grounded at the emitter thereof and connected at the collector thereof to an output terminal 2 and another NPN-type transistor 3 which is connected in a diode form. The base of the transistor 1 is connected to the connection point between the collector and base of the transistor 3, i.e., the anode of the diode, and the emitter of the transistor 3, i.e., the cathode of the diode is grounded. The connection point between the collector and base of the transistor 3 is connected through a resistor 4 to a power source terminal 5 which is supplied with a positive DC voltage V.sub.CC so that a constant current I.sub.C flows to the transistor 1. In this case, if the emitter area of the transistor 1 is selected equal to that of the transistor 3 which is connected as a diode, the base-emitter voltage of the transistor 1 is taken as V.sub.BE and the resistance value of the resistor 4 is taken as R.sub.1, respectively, the constant current I.sub.C is expressed as follows: EQU I.sub.C =1/R.sub.1 (V.sub.CC -V.sub.BE) (1)
A prior art current mirror circuit is shown in FIG. 2, in which reference numerals are the same as those used in FIG. 1 to represent the same elements. The emitter of the NPN-type transistor 1 is grounded through a resistor 6, the base thereof is connected through a resistor 7 to the connection point between the collector and base of the transistor 3 which is connected in the form of a diode and is grounded at the emitter thereof and the base of the transistor 1 is connected through the resistor 4 to the power source terminal 5 which is supplied with the DC voltage of V.sub.CC similar to the example of FIG. 1, so that the constant current I.sub.C flows to the transistor 1. In this case, if the transistors 1 and 3 are selected equal in their emitter area and the resistance values of the resistors 7 and 6 are taken as R.sub.2 and R.sub.3, respectively, the constant current I.sub.C can be expressed as follows: ##EQU1##
As may be apparent from the above equations (1) and (2), the constant current I.sub.C from the prior art constant current output circuits or current mirror circuits shown in FIGS. 1 and 2 is in proportion to (V.sub.CC -V.sub.BE). Accordingly, in the constant current output circuits shown in FIGS. 1 and 2, if the condition V.sub.CC &gt;V.sub.BE is satisfied, variation or fluctuation of the constant current I.sub.C caused by fluctuation of V.sub.BE of the transistor 1 depending upon temperature change can be neglected and hence it can be said that the constant current I.sub.C has no temperature characteristic.
However, when the power source voltage V.sub.CC is low and accordingly is not as high as compared with the base-emitter voltage V.sub.BE of the transistor 1, the output constant current I.sub.C depends upon the voltage V.sub.BE or is changed in accordance with the temperature characteristic thereof. Therefore, the prior art constant current output circuits shown in FIGS. 1 and 2 can not be said to be a stabilized constant current output circuit. In other words, when the power source voltage V.sub.CC becomes low, the prior art constant current output circuits shown in FIGS. 1 and 2 lose the constant current characteristics and hence can not be used practically.